In addition to upgrading legacy networks and equipment, mobile service providers are also looking for ways to offload the spectrum through the rollout of high-bandwidth indoor and outdoor wireless networks. One approach for spectrum offload is the deployment of multiple small cells in the network. Small cells (which comprise femtocells, picocells, metrocells and microcells) are low-power wireless access points that operate within the licensed spectrum.
Used primarily in residential and enterprise business settings, small cells communicate with the carrier network through a broadband connection as shown in FIG. 1, allowing users to continue using their mobile devices without an interruption in connectivity. Small cells are ideally suited to deliver improved coverage, capacity and signal strength in these environments and offer a relatively seamless approach for carriers to deliver greater capacity and faster data speeds while offloading traffic from the spectrum.
By delivering service using small cells, the operator is ensuring that the user is closer to the transmitter and enjoying the best possible performance. Finally, small cells provide cost-effective base station design and deployment. When deployed in a residential area or home use situation, the operator no longer bears the cost of site acquisition, power or backhaul from the customer premise to the headend or aggregation point.
Small cells require, in particular, an economical and, preferably, a readily available backhaul solution to serve the large number of small cells. They require access to the rest of the mobile infrastructure such as macro-cells, clock references and mobile network controllers. Leveraging the existing broadband (e.g., DSL, GPON) infrastructure to the customer premises (FIG. 1) for small cells backhaul conceivably can lead to lower access costs than traditional leased line services. DSL, for example, can be a high-performance and cost-effective solution for mobile backhaul. Benefits of DSL for mobile backhaul include:                Ubiquitous availability        Reuse of copper resources in the network        Simple installation and easy maintenance        Reduction in Operational Expenditure (“OpEx”) and Capital Expenditure (“CapEx”) compared with other alternatives        
DSL equipped with time synchronization has therefore become an appealing alternative for mobile backhaul.
Time Synchronization of Small Cells
Synchronization plays a crucial role in mobile backhaul networks. Time Division Duplexing (TDD) small cells require a phase difference of less than 3 μs between base stations of adjacent cells. This is normally achieved by ensuring that the base stations are synchronized to within ±1.5 μs. 4G/LTE-Advanced, in particular, requires even stricter clock distribution accuracy at all base stations to ensure support for new features like Coordinated Multi-Point (CoMP) operation, enhanced Inter-Cell Interference Coordination (eICIC), Carrier Aggregation, and Location Based Services (LBS). The accuracy required by each of these techniques has not yet been agreed by 3GPP.
Synchronization may be provided to small cells by distributing a time reference over the backhaul network to the small cells. IEEE 1588 Precision Time Protocol (PTP) [1] is now the industry accepted packet-based method/standard for distributing timing information from a master to enable the clocks of distributed systems to be synchronized with high precision (accuracies in the nanosecond levels). It is also designed for applications that cannot bear the cost of a GPS receiver plus antenna at each node, or for which GPS signals are inaccessible. Installing a GPS antenna on every cellular base station has consequences in terms of both CapEx and OpEx. It can also add more complexity to the initial installation process of the base station (additional antennas, wiring, receiver, technician time whenever the outdoor antenna requires maintenance, etc.).
The delivery of accurate synchronization to small cell base stations is one of the major challenges facing mobile network operators. One existing approach for bigger base stations like macrocells is to co-locate a GPS receiver and antenna at each base station. But this will not be economical for small cell networks, which most likely will see an exponential growth in the number of cell sites, each covering a relatively small slice of the area covered by a single macrocell base station. Moreover, small cell base stations are much smaller than macrocell base stations, and in many cases they will be unable to host a GPS receiver and antenna, either because there is no space or because there is no clear sky view to a GPS satellite, or both. Also, the issue is simply a matter of economics because purchasing, installing and maintaining a GPS receiver and antenna at every small cell base station will quickly become prohibitively expensive. Furthermore, small cells deployed in the indoor environment are unlikely to be able to acquire a GPS signal, due to the attenuation of the signal caused by the building in which they are housed, and will need to use a network-delivered reference. Hence, transferring timing via packet transport is a relevant synchronization technique that must coexist and interoperate with all others.
In addition to oscillator stability, the quality of the time synchronization depends heavily on how well the level of packet delay variations (PDVs), communication path asymmetries, and system noise between the clock source (master) and the receiver (slave) are mitigated. An additional source of communication path asymmetry (that exists in technologies such as DSL and GPON) is transmission rate asymmetry which occurs when the forward and reverse paths have different transmission rates resulting in a transmission delay asymmetry. Transmission delay asymmetry further complicates the problem of clock recovery at the end-user (slave).
Therefore, one of the main challenges in clock transfer over packet networks is dealing with the packet delay variation (PDV) and communication path asymmetries (i.e., asymmetry in forward and reverse path delays) in the network, which can present considerable error components in the recovered clock. These error components have to be adequately mitigated in order to obtain accurate clock performance. The communication path asymmetries can be due to one or more of: physical link (wire/fiber) asymmetries; internal device signal path and processing delay asymmetries (chip, board, and board-to-board level delays); and load (or queuing) induced asymmetries (which arise when the queuing delays in the forward and reverse paths are different).
An additional source of communication path asymmetry is transmission rate asymmetry which occurs when the forward and reverse paths have different transmission rates resulting in a transmission delay asymmetry. The transmission delay (which is inversely proportional to the transmission rate) is the amount of time it takes to put data on the transmission medium. The higher the transmission rate, the shorter the transmission delay.
Time transfer using a protocol such as IEEE 1588 PTP and a well-designed slave clock recovery mechanism can provide time synchronization in the sub-microsecond level and lower. However, this is usually done using the important assumption that the message transfer delay from master to slave is equal to that from slave to master. In real life, the communication paths are not perfectly symmetric, mainly due to dissimilar forward and reverse physical link delays, internal device delays, and queuing delays. Even in cases where the physical link and internal device delays are known and properly compensated for during clock synchronization, queuing delays which are variable can still exist when timing messages go through the packet network and queued for forwarding.
In some variants of DSL and GPON, for example, the inherent transmission rate asymmetry is additional source of delay asymmetry that needs to be properly addressed during clock synchronization.
Overview of IEEE 1588v2 PTP
The IEEE 1588v2 PTP defines a packet-based synchronization protocol for communicating frequency, phase and time-of-day information from a master to one or more slaves with sub-microsecond accuracy. PTP relies on the use of accurately timestamped packets (at nanosecond level granularity) sent from a master clock to one or more slave clocks to allow them to (frequency or time) synchronize to the master clock. Synchronization information is distributed hierarchically, with a GrandMaster clock at the root of the hierarchy. The GrandMaster provides the time reference for one or more slave devices. These slave devices can, in turn, act as master devices for further hierarchical layers of slave devices. PTP provides a mechanism (i.e., Best Master Clock Algorithm) for slave clocks to select the best master clock in their respective synchronization domain. The selection is performed according to the PTP attributes of the GrandMaster (e.g. PTP priority, clock class).
The PTP message exchange process (i.e., the PTP Delay Request/Delay Response flow) between a master and a slave is performed as follows and as illustrated in FIG. 2. IEEE 1588 PTP allows for two different types of timestamping methods, either one-step or two-step. One-step clocks update time information within event messages (Sync and Delay-Req) on-the-fly, while two-step clocks convey the precise timestamps of packets in general messages (Follow_Up and Delay-Resp). A Sync message is transmitted by a master to its slaves and either contains the exact time of its transmission or is followed by a Follow_Up message containing this time. In a two-step ordinary or boundary clock, the Follow_Up message communicates the value of the departure timestamp for a particular Sync message.
FIG. 2 illustrates the basic pattern of synchronization message exchanges for the two-step clocks. The master 1 sends a Sync message to the slave 3 over the packet network 2 and notes the time T1 at which it was sent according to the master clock 4. The slave 3 receives the Sync message and notes the time of reception T2 according to the slave clock 5. The master 1 conveys to the slave the timestamp T1 by one of two ways: 1) Embedding the timestamp T1 in the Sync message (one-step clock—not shown). This requires some sort of hardware processing (i.e., hardware timestamping) for highest accuracy and precision. 2) Embedding the timestamp T1 in a Follow_Up message (two-step clock—as shown in FIG. 2). Next, the slave 3 sends a Delay_Req message to the master 1 over the packet network 2 and notes the time T3 at which it was sent according to the slave clock 5. The master 1 receives the Delay_Req message and notes the time of reception T4 according to the master clock 3. The master 1 conveys to the slave 3 the timestamp T4 by embedding it in a Delay_Resp message.
At the end of this PTP message exchange, the slave 3 possesses all four timestamps {T1, T2 , T3 , T4}. These timestamps may be used to compute the offset of the slave clock 5 with respect to the master clock 4 and the communication delay of messages between the two clocks. The computation of offset normally assumes that the master-to-slave and slave-to-master path delays are equal, i.e. a symmetrical communication path. Clock frequencies change over time, so periodic message exchanges are required. Because these clock variations change slowly, the period between message exchanges is typically on the order of milliseconds to seconds.
Linear Programming
A number of linear programming methods have been proposed for clock synchronization and network measurements [2][3][4][5][6]. All of these methods have been formulated on the assumption that the data transmission rates, that is, the downstream and upstream transmission rates, between the master and slave are equal (symmetric). Equivalently, this means the transmission delays between the two endpoints are equal. All other delays such as the propagation delays and queuing delays (which cause (PDVs)) in the two directions do not necessary need to be equal. However any asymmetry in the propagation delays has to be known and compensated for during clock synchronization.
Backhaul technologies such as DSL or GPON, are inherently asymmetrical in transmission rates, and thus will require some form of transmission rate asymmetry compensation (in addition to other existing path asymmetries) in order to deliver accurate time synchronization. Thus, if time synchronization has to be delivered over such backhaul networks with asymmetrical delay on upstream and downstream, the level of asymmetry has to be accurately identified and proper compensation must be applied to avoid a time offset at the packet slave clock.
The transmission rate asymmetry issue (in addition to other path asymmetries) in DSL and GPON makes the distribution of synchronization signals from a central GrandMaster (GM) located somewhere in the core network right to the small cells attached to the head-end or aggregation point (e.g., DSLAM) problematic. End-to-end (E2E) time transfer by itself even over communication paths with no transmission rate asymmetry issues is challenging. Adding transmission rate asymmetry in this E2E time transfer scenario makes the problem even much more challenging. For these reasons, a much more acceptable approach is to incorporate or collocate a boundary clock (BC) or GM at the headend or aggregation point (i.e., DSLAM) which most likely will be located in the exchange (see FIG. 1). A more advanced solution could even have the GM or BC incorporated into headend or aggregation point device and also equipped with a built-in GPS receiver.
An object of the present invention is to recover the master clock accurately at the slave in spite of the delays caused by the network connection. In particular it is an object of the present invention to take account of transmission asymmetry issues in the network to improve the accuracy of the clock recovery.